The government's legal theory in joining the lawsuit is that when Armstrong agreed to race for the US Postal Service team a decade ago in the Tour de France, he defrauded the government, violating its strict ban on illegal drugs, all the while claiming he did not use them.

Though the government's action presents a serious new legal threat to Armstrong, the Justice Department case is not foolproof. Legal experts say Armstrong could argue that his contract with the team owners never explicitly prohibited blood doping. And he could claim that he never signed any agreement directly with the Postal Service that banned the practice.

But if the government wins, Armstrong could face huge fines, because the Postal Service paid at least $30 million to sponsor his racing teams.

The cycling website Velo News reported this week that Travis Tygart, the CEO of the US Anti-Doping Agency, wrote to Attorney General Eric Holder last month, urging the government to join the Landis lawsuit.

A decision by the Justice Department to join the case "in order to get to the bottom (or top) of this massive fraud would also be viewed by the press and public as necessary and legitimate," the letter said.